1. Technical Field
This invention relates to a heater control apparatus for use in laminaters or the like.
2. Description of the Prior Art
FIG. 13 shows a conventional heater control apparatus, wherein reference numeral 1 denotes a temperature control circuit, 2 denotes a power element consisting of a both-way conducting element, such as a triac, 2A denotes an AC power supply, 3 denotes a heater heated by a current applied from the AC power supply 2A through the power element 2, and 4 denotes a temperature detection element, such as a thermister for detecting the temperature of the heater 3 or an object to be heated by the heater 3.
In such conventional heater control apparatus, the temperature control circuit 1 receives a detected temperature signal a from the temperature detection element 4 and outputs an exciting current control signal b for controlling the power element 2. The power element 2 is controlled by the exciting current control signal b and the heater 3 is heated.
The temperature control circuit 1 comprises a detected temperature processing circuit 5 which receives the detected temperature signal a from the temperature detection element 4 and outputs a detected temperature processing signal c of a predetermined level, a temperature setting device 6 for setting a temperature of the heater 3 or the object heated by the heater 3, a comparator 7 for comparing a temperature setting signal d outputted from the temperature setting device 6 with the detected temperature processing signal c outputted from the detected temperature processing circuit 5, and a zero crossing type power element driver 8b for receiving a control signal e from the comparator 7 and generating the exciting current control signal b, wherein the temperature of the heater 3 or the object heated by the heater 3 is maintained at the temperature set by the temperature setting device 6.
The zero crossing type power element driver 8b comprises a photo triac coupler and resistors as shown in FIG. 13, for example.
R1 and R2 shown in FIG. 13 denote resistors.
FIG. 14 shows a timing chart of an operation of the conventional heater control apparatus, wherein hatched wave form portions in FIG. 14(a) show a heater current and solid line portions show an input voltage. In FIG. 14(b), a reference symbol c designates the detected temperature processing signal, and d designates the temperature setting signal. In FIG. 14(c), a reference symbol e designates the control signal. In FIG. 14(d), a reference symbol b designates the exciting current control signal.
The function of the conventional heater control apparatus will now be explained with reference to FIG. 14.
When the detected temperature processing signal c is lower in level than the temperature setting signal d, as shown in FIG. 14(b), the control signal e outputted from the comparator 7 is low level as shown in FIG. 14(c), so that positive pulses and negative pulses of the exciting current control signal b are generated at such a timing that the input voltage (sine wave) applied by the AC power supply 2A becomes zero, as shown in FIG. 14(d).
The power element 2 receives the positive and negative pulses of the exciting current control signal b and supplies the maximum heater current indicated by the hatched portions in FIG. 14(a) to the heater 3 for a period of time from t1 to t2 and a period of time from t3 to t4.
In a period of time from t2 to t3, the detected temperature processing signal c is higher in level than the temperature setting signal d as shown in FIG. 14(b), and the control signal e outputted from the comparator 7 becomes high level (H) as shown in FIG. 14(c), so that the zero crossing type power element driver 8b maintains the exciting current control signal b at zero volt.
The power element 2 receives the-exciting current control signal b maintained at zero volt, and stops the current supply to the heater for the period of time from t2 to t3 as shown in FIG. 14(d).
By repeating the above operations, the temperature of the heater 3 or the object heated by the heater 3 can be controlled to a temperature corresponding to the temperature setting signal d set by the temperature setting device 6.
However, the detected temperature signal a detected by the temperature detection element 4 fluctuates in a range due to the time lag of the thermal transmission between the heating portion of the heater 3 and the temperature detection element 4.
The range of fluctuation is varied according to the input voltage and becomes wide when the input voltage becomes high, so that the average temperature becomes high.
Accordingly, it is necessary to adjust the temperature setting signal d set by the temperature setting device 6 so that the mean value of the detected temperature signal a in case that an input voltage of the lowest level is applied is equal to the mean value of the detected temperature signal a in case that an input voltage of a level other than the lowest level is applied, in the conventional heater control apparatus to which at least two levels of input voltage can be applied.
Further, in the prior art, the range of the fluctuation of the detected temperature signal a cannot be adjusted.